PROJECT SUMMARY/ABSTRACT In this application we describe the development of an ultra-high speed Serial Two-Photon (STP) Tomography microscope that can image entire organs with micron resolution in less than a day. It will offer the highest imaging speed and sensitivity available for fluorescent subcellular whole organ imaging. We will accomplish this by implementing a novel multi-foci multiphoton microscope (MMM) version of our current STP system that builds on TissueVision?s founding team?s extensive experience with MMM systems. It will address the two major problems with existing MMMs: 1. Limited field of view due to aberrations induced by the intermediate optics 2. Loss of emission photons in the descanned path. We will present details of a new excitation scheme that avoids aberrations and restricted field of views, and describe a non-descanned detection scheme that employs a high efficiency fiber coupled detection with an original interlaced scanning strategy that minimizes foci crosstalk. This proposal overcomes several long standing technical problems with MMM systems for deep tissue imaging, with a novel MMM design ideally suited for ex vivo whole organ imaging. The ability to fluorescently image a whole organ in 3D at 1 micron XY sampling and 2 micron Z sampling in less than 12 hours and will have a transformative effect on 3D histology and provide a crucial tool for researchers for a vast array of applications in neuroscience and other fields.